A time-pressure liquid dispenser usually delivers pressurized air to dispense a required amount of liquid from a syringe. Proper control of the dispensing duration and dispensing pressure determine the dispensing performance, such as the volume consistency of the fluid dispensed. Typically, measurement of dispensing pressure is used as a means for adjusting the dispensing time to dispense a predetermined amount of liquid. The time-pressure liquid dispenser generally comprises a syringe containing a liquid for dispensing, an air source for supplying air under positive pressure and a dispensing solenoid valve located between the syringe and the air source. Conventionally, two pressure sensors are used for monitoring pressure in the liquid dispenser. A first pressure sensor measures a syringe pressure and a second pressure sensor measures a source pressure of air from the air source. A controller receives signal inputs of measured pressure from the first and second pressure sensors and discharges a voltage signal output for actuating the dispensing solenoid valve to supply a positive air pressure from the air source to the syringe in order to discharge the liquid from the syringe. An air suction device provides negative air pressure to the syringe for preventing the liquid from dripping from the syringe after completion of the dispensing operation.
The controller typically controls the duration of the output voltage signal until the syringe pressure reaches a preset value depending on the amount of liquid remaining in the syringe. Variation in the syringe pressure due to the amount of liquid remaining in the syringe is estimated by the controller to keep the amount of liquid dispensed constant regardless of the amount of liquid remaining in the syringe. Hence, the apparatus is controlled to dispense liquid from the syringe until the air pressure in the syringe reaches the preset pressure. However, accurate dispensing of the liquid may not be possible when the air pressure does not reach the preset pressure at the correct time due to change or variation of the syringe inner volume, conduit inner diameter or compressed pressure.
U.S. Pat. No. 5,277,333 entitled “Apparatus for Metering and Discharging a Liquid” discloses a liquid dispenser capable of dispensing a constant amount of liquid in spite of variations of air pressure in a syringe. The dispenser consists of two pressure sensors, three valves and corresponding pneumatic and electronic circuits. A first pressure sensor detects and measures source pressure from an air source while a second pressure sensor detects and measures syringe pressure. A first valve switchably connects pressurized air from the air source to the syringe to dispense liquid, or releases pressure from the syringe to stop dispensing. A second valve switchably connects the syringe to a vacuum source and a third valve switchably connects the syringe to an exhaust vent. A controller generates a shifting output signal for a certain duration corresponding to a quantity of liquid remaining in the syringe so as to accurately dispense the liquid from the syringe.
However, there are disadvantages in the design of this liquid dispenser. Since the first valve either connects the syringe to receive pressurized air or to release air, pressurization and release of air cannot be conducted simultaneously. This slows down liquid dispensation and more importantly, does not provide real time control of the syringe pressure which may reduce the accuracy of the syringe pressure measured. The accuracy of the determined dispensing time may also be affected. It is therefore desirable to regulate the pressure more effectively so as to be able to dispense a more precise amount of liquid. Furthermore, only two pneumatic pressures can be measured, namely the source pressure and the syringe pressure. Measuring more than just the two pneumatic pressures would be beneficial to improve general dispensing performance. Moreover, adjustment of the pressure of the pressurization air source is done manually off-line. This gives rise to idle time and lengthens the operation time.